Linear electrooptic switching and second-harmonic generation are processes that require materials possessing second-order nonlinear susceptibilities. That is, materials for which the polarization of the material has a term that responds to the square of the electric field. For optical switching, an applied DC or low frequency field can then modulate the material response to an optical field leading to optical switching. In second harmonic generation, the polarization at the second harmonic is proportional to the square of the applied optical field at the fundamental (first harmonic) frequency. It has long been known that for materials with inversion symmetry, i.e. materials that cannot in some way distinguish between up and down, the second-order susceptibility must vanish. Even though materials such as silica (SiO.sub.2) may have a crystal structure without inversion symmetry, the random orientations found in amorphous or glassy forms of the material ensures a macroscopic inversion symmetry.
There are myriad applications for materials with second-order nonlinearities, particularly in integrated optics and optoelectronics. For example, LiNbO.sub.3 waveguide switches for optical crossbar applications are commercially available. As optoelectronics matures with widespread applications in optical information processing, there is a growing need for the development of improved high-speed waveguide switches, directional couplers, and other signal routing devices. Compatibility with semiconductor optoelectronics will lead to a major simplification and growth of the market for these devices. LiNbO.sub.3 switches are based on a bulk crystal technology, which is not directly compatible with semiconductors such as Si and GaAs or other III-V compounds that are widely used for the fabrication of laser sources and detectors in optoelectronics.
There is also increasing interest in harmonic generation in waveguides, particularly to convert the infrared output of III-V semiconductor lasers to the visible. Here again compatibility with traditional semiconductor materials is a major issue. Present techniques rely principally on external harmonic generation, usually involving resonant cavity effects, with relatively complex optical and mechanical arrangements.
Fused silica is ubiquitous in modern technology. Its extremely low linear optical losses have enabled the fiber optics industry. SiO.sub.2 also plays a dominant role in microelectronics technology where the unique properties of the Si-SiO.sub.2 interface are largely responsible for the behavior of metal-oxide semiconductor (MOS) devices underlying advances in computer hardware.
Unlike its related quartz crystalline phase, fused silica is amorphous with a macroscopic inversion symmetry that forbids second-order nonlinear processes. Thus, the discovery by U. Osterberg and W. Margulis, Opt. Lett. 11, 516 (1986) of efficient second harmonic generation (SHG) in a variety of Si-Ge glass fibers upon "training" with optical fields has generated considerable interest in the physics and applications of this unexpected phenomenon. R. H. Stolen and H. W. K. Tom, Opt. Lett. 12, 587 (1987) showed that the nonlinearity could be induced in a much shorter time by seeding the fiber simultaneously with both the fundamental optical (1.06 .mu.m) and the second harmonic (532 nm) beams and proposed a mechanism based on static electric-field-induced nonlinearities. The field arises from charge separation and trapping in the fiber material. Recently, D. Z. Anderson, V. Mizrahi and J. E. Sipe, Opt. Lett. 16, 796 (1991) have proposed a photovoltaic effect based on interference between the fundamental and harmonic fields that phenomenologically accounts for the observed strength of this field, which (see A. Kamal, M. L. Stock, A. Szpak, C. H. Thomas, D. A. Weinberger, M. Frankel, J. Nees, K. Ozaki, and J. Valdmanis, in Digest of Optical Society of America Annual Meeting (Optical Society of America, Washington, D.C., (1990) paper PD25) is about four orders of magnitude larger than the field expected from optical rectification. This field interacts with the material third-order nonlinearity, .chi..sup.(3), to provide an effective .chi..sup.(2) =.chi..sup.(3) E.sub.dc. Similar field-induced nonlinearities have been observed in a variety of material systems, e.g. paraelectric PLZT by A. Mukherjee, S. R. J. Brueck and A. Y. Wu, Opt. Commun. 76, 220 (1990). An alternative explanation is an orientation of nonlinear moieties in the glass, although no microscopic indentification of these moieties or of the magnitude of their nonlinearities has been put forward.